This invention relates to a process of purifying flue gases or other humid exhaust gases, which contain NO.sub.x and SO.sub.2, wherein the NO.sub.x content is reduced and the SO.sub.2 content is oxidized to SO.sub.3 by a catalytic processing and the SO.sub.3 content is subsequently condensed as sulfuric acid.
The pollutants contained in flue gases from fuel-firing furnaces mainly consist of NO.sub.x, SO.sub.2, HCl and HF. For instance, the combustion of coal which contains 1% by weight sulfur in large fuel-firing plants will result in a flue gas which contains 6% by volume oxygen and contains per sm.sup.3, e.g., 1900 to 2000 mg SO.sub.2, less than 150 mg HCl and less than 50 mg HF.
In dependence on the fuel-firing process and on the coal which is combusted the NO.sub.x contents may range from less than 600 mg/sm.sup.3 to more than 2000 mg/sm.sup.3.
It is known that the emission of such polluants can be decreased in that the flue gas is subjected to a catalytic aftertreatment in which the NO.sub.x is reduced with NH.sub.3 to N.sub.2 and the SO.sub.2 is oxidised to SO.sub.3. The humid gas is subsequently cooled below the dew point temperature of sulfuric acid and SO.sub.3 is condensed as sulfuric acid.
Such a process is known from Published German Application 33 31 545. The SO.sub.2 -containing hot flue gas is dedusted in an electrostatic precipitator and is then cooled to the temperature which is required for the catalysis. Thereafter the SO.sub.2 is catalytically reacted with oxygen to SO.sub.3. The SO.sub.3 -containing humid gas is cooled in a first stage of an air preheater to a temperature above the dew point temperature of sulfuric acid and is subsequently cooled in a second stage to a temperature which is below the dew point temperature of sulfuric acid and at which the flue gas may be admitted to the chimney. The second stage of the air preheater is made of acid-resisting material, such as glass tubes.
In the process described in U.S. Pat. No. 4,164,546 the hot flue gases are suitably cooled in an economizer or air preheater and are then catalytically treated with an addition of ammonia. The oxidation catalyst may consist of a catalyst based on V.sub.2 O.sub.5 and K.sub.2 O. The SO.sub.3 which has been formed is separated in known manner.
In the process described in Published German Application 36 01 378 the flue gases are first catalytically treated with an addition of ammonia to reduce the NO.sub.x and SO.sub.2 is then catalytically reacted with oxygen to form SO.sub.3. After the catalytic processing an interstage cooling with water is effected and the SO.sub.3 is reacted to form sulfuric acid. The purified gas is fed to the chimney. The flue gases are dedusted in an electrostatic precipitator at an elevated temperature before the catalysis or in a dust filter after the catalysis.
Although the flue gases are dedusted in said processes, they still contain residual dust, which may clog or deteriorate the catalyst and may introduce impurities into the condensed sulfuric acid.